Programming: Part I. In this section of notes you will learn about how to write simple programs using JES. Translators

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1 Programming: Part I In this section of notes you will learn about how to write simple programs using JES. Translators The (greatly simplified) process of writing a computer program. Step 3: The (binary) program can directly be executed/run. # The Simpsons Game while (gamestatus == PLAYING): display (amaze, artictime, currentrow, gamestatus) processmovement (amaze) currentrow, currentcolumn) Executable program Step 1: A programmer writes a program in a human understandable form. Step 2: A translator converts the program into a computer understandable form

2 What Your Will Be Writing/Translating Your Programs With In This Class Writing programs this semester: Programming language: Python (actually it s a modified version of Python called JPython or the JES version of the Python programming language ) - Starting tutorial: - Online documentation: - My old CPSC 217 notes: Software to write/translate your Python programs: JES - Quick introduction: Where You Can Run JES Lab computers: JES is installed and running on all the computers in both 203 labs Installing on your own computer (Windows, Mac, Linux) 1 : OR 1 Java is needed run JES so if you don t have it installed then you should download it as well

3 Resources My completed example programs can be found on the course web page under the following link: Getting Started At Home 1 Step 1: Download the version appropriate to your computer (Windows, Mac, Linux). Uncompress the compressed (zip format) file using the appropriate program (in Windows it s built into the operating system). Right click on the downloaded file: 1 Note: It is NOT required for this course that you install JES at home. No warranties or guarantees of service are provided for this program (i.e., we aren t responsible if you inadvertently damage your computer during the installation process).

4 Getting Started At Home (2) Pick a location to extract the files to that you will remember: Note: to keep it simple any data files (e.g., images) that you need for your programs should be stored in this folder/directory. Getting Started At Home (3) To start the program click on the JES icon:

5 Once JES Has Been Started (Home Or In The Lab) The splash screen will first load (it may stay on a few seconds even with a fast computer) JES Built in help for using JES and for some programming concepts in JPython Type in your programs here Type in your commands (e.g., run the program that you just entered) here

6 File Operations Similar to a text editor (e.g., notepad) and are used in conjunction with the creation of your programs (load, save, print etc.) The Basic Structure Of A Program In JES Format: def program name (): Body of the program 1 Indent the body using three spaces (or at least make it consistent throughout). Example: def start (): print Starting my first program! 1 This is the part that actually completes actions such as: displaying messages onscreen, loading a picture from file, performing a calculation etc.

7 Creating And Running Your First Program Step 1: Type in your program in the editing area: Creating And Running Your First Program (2) Step 2: Load your program so it can be translated into binary. (Currently your program has been loaded into the editor but not loaded into the JES translator). The file menu affects the editor and not the translator (e.g., open doesn t load the program) Load your program here Program hasn t been loaded yet

8 Creating And Running Your First Program (2) Step 3: When you run your program JES will ask if you want to save it. Save it with a file name that ends in dot-py (e.g., start.py) Creating And Running Your First Program Step 4: Run your program in the command area IMPORTANT: the name that you type in to run the program must match the program name specified here Running program called start The effect of running your program is displayed immediately after you run it.

9 Storing Information When writing a program there will sometimes be a need to store information e.g., to perform a calculation. Information is stored in a computer program by using a variable. Variables Set aside a location in memory Used to store information (temporary) This location can store one piece of information At most the information will be accessible as long as the program runs Some of the types of information which can be stored in variables: Integer (num = 10) Real numbers (num = 10.5) Strings (message = Not happy! >-< ) Picture from Computers in your future by Pfaffenberger B

10 Arithmetic Operators Operator = + - * / % ** Description Assignment Addition Subtraction Multiplication Division Modulo Exponent Example num = 7 num = num = 6-4 num = 5 * 4 num = 25 / 5 num = 8 % 3 num = 9 ** 2 Displaying Output When there s a need to display information onscreen: Status messages or instructions Contents of variables

11 Displaying String Output Displays information or messages in the command area of JES. Example: Print: A command to display information In this case, everything between the quotes will appear in the command area (string = series of characters). Displaying String Output Format: print <string of characters> Example: def outputexample (): print Please don t display this message!

12 Format: Displaying The Contents Of Variables print <name of variable> Example: def example1 (): num = 10 print num Displaying Mixed Output Strings and the contents of variables can be intermixed with a single print statement. Format: print <string>, <variable name>... Example: Available online and is called profit.py : def profit (): income = 2000 expenses = 1500 profit = income - expenses print "Income: ", income, " Expenses: ", expenses, " Profit: ", profit

13 Working With Picture Variables One of the strengths of JES is the ease at which multimedia files can be incorporated in a computer program. Example: Available online and is called picture1.py, requires that you also download and save the image called lion.jpg to the folder where you are running JES from). def picture1(): picture = makepicture ("lion.jpg") show (picture) Getting Input In JES it can be done as the program runs. Example: How to specify the name of the images as the program runs. (Available online and is called picture2.py ): def picture2 (file1,file2): picture1 = makepicture(file1) show(picture1) picture2 = makepicture(file2) show(picture2) To run this program you must enter the name of two images as you run the program in the command area E.g., picture2("angel.jpg","valerie.jpg")

14 Getting A File Dialog Box Example: Available online and is called picture3.py def picture3(): filename = pickafile() mypicture = makepicture (filename) show (mypicture) What To Do If A Program Needs To Choose Among Alternatives? Employ branching/decision-making Each alternative involves asking a true/false (Boolean) question The answer to the question determines how the program reacts

15 Example Of The Need For Branching Is income below $10,000? Nominal income deduction Eligible for social assistance True False True Income tax = 20% False etc. Is income between $10K - $20K? Some Types Of Branching Mechanisms In JES If If-else Compound decision making Nested decision making

16 To be used when a true/false question (Boolean expression) is asked and: The program reacts one way if the question evaluates to true Example: If person is a senior citizen then give a 25% discount. If Decision Making With An If Question? True Execute a statement or statements False Remainder of the program

17 The If Decision making: checking if a condition is true (in which case something should be done). Format: if (Boolean expression): body Note: Indenting the body is mandatory! The If (2) Example: Available online and is called ifexample1.py def ifexample1 (age): if (age >= 18): print "Adult" print "Tell me more"

18 Types Of Comparisons Python Mathematical operator equivalent Meaning Example < < Less than 5 < 3 > > Greater than 5 > 3 == = Equal to 5 == 3 <= Less than or equal to 5 <= 5 >= Greater than or equal to 5 >= 4 <> Not equal to 5 <> 5 OR!= 5!= 5 If (Simple Body) Body of the if consists of a single statement Format: Indenting used to indicate what statement is the body if (Boolean expression): s1 s2 Example: if (num == 1): print Body of the if print After body Body

19 Body of the if consists of multiple statements Format: if (Boolean expression): s n+1 s 1 s 2 : s n If (Compound Body) Body End of the indenting denotes the end of decision-making If (Compound Body(2)) Example: Available online and is called ifexample2.py def ifexample2 (income): taxcredit = 0 taxrate = 0.2; if (income < 10000): print Eligible for social assistance taxcredit = 100 tax = (income * taxrate) taxcredit print tax

20 Decision Making With An If : Summary Used when a question (Boolean expression) evaluates only to a true or false value (Boolean): If the question evaluates to true then the program reacts differently. It will execute a body after which it proceeds to execute the remainder of the program (which follows the if construct). If the question evaluates to false then the program doesn t react different. It just executes the remainder of the program (which follows the if construct). If-Else To be used when a true/false question (Boolean expression) is asked and: The program reacts one way if the question evaluates to true The program reacts another way if the question evaluates to false Example: If income is greater than $10,000 then calculate taxes owed, otherwise calculate subsidy to be given back to the person.

21 Decision Making With An If-Else Question? True Execute a statement or statements False Execute a statement or statements Remainder of the program The If-Else Decision making: checking if a condition is true (in which case something should be done) but also reacting if the condition is not true (false). Format: if (Boolean expression): body of 'if' else: body of 'else' additional statements

22 If-Else Construct (2) Example: Available online and is called ifexample3.py def ifexample3 (age): if (age >= 18): print Adult else: print Not an adult print Tell me more about yourself If-Else (Compound Body(2)) Example: Available online and is called ifexample4.py def ifexample4 (income): taxcredit = 0 if (income < 10000): print Eligible for social assistance taxcredit = 100 taxrate = 0.1 else: print Not eligible for social assistance taxrate = 0.2 tax := (income * taxrate) taxcredit print tax

23 Quick Summary: If Vs. If-Else If: Evaluate a Boolean expression (ask a question) If the expression evaluates to true then execute the body of the if. No additional action is taken when the expression evaluates to false. Use when your program evaluates a Boolean expression and code will be executed only when the expression evaluates to true. If-Else: Evaluate a Boolean expression (ask a question) If the expression evaluates to true then execute the body of the if. If the expression evaluates to false then execute the body of the else. Use when your program evaluates a Boolean expression and different code will execute if the expression evaluates to true than if the expression evaluates to false. Decision-Making With Multiple Expressions Format: if (Boolean expression) logical operator (Boolean expression): body Example: if (x > 0) and (y > 0): print X is positive, Y is positive

24 Decision-Making With Multiple Expressions (2) Commonly used logical operators in Python or and not Forming Compound Boolean Expressions With The OR Operator Format: if (Boolean expression) or (Boolean expression): body Example: if (gpa > 3.7) or (yearsjobexperience > 5): print You are hired

25 Format: Forming Compound Boolean Expressions With The AND Operator if (Boolean expression) and (Boolean expression): body Example: if (yearsonjob <= 2) and (salary > 50000): print You are fired Forming Compound Boolean Expressions With The NOT Operator Format: if not (Boolean expression): body Example: if not (x == 0): print X is anything but zero

26 Nested Decision Making Used when there s a need to qualify questions. One or more questions are asked only if another question evaluates to true. Example: Looking for people to hire Question 1: Did the candidate complete an advanced graduate degree (masters or doctoral degree) - (The following questions are asked only if the person does have a graduate degree) - Question 1A: Is the grade point 3.7 or above. - Question 1B: Does the person have 5 or more years of work experience. This is referred to as nested decision making because some questions are dependent on the answer to other questions (dependent questions are nested inside another question). Nested Decision Making Decision making is dependent. The first decision must evaluate to true before successive decisions are even considered for evaluation. Question 1? True Question 2? True Statement or statements False False Remainder of the program

27 Nested Decision Making One decision is made inside another. Outer decisions must evaluate to true before inner decisions are even considered for evaluation. Format: if (Boolean expression): if (Boolean expression): inner body Outer body Inner body Nested Decision Making (2) Example: Available online and is called ifexample5.py def ifexample5 (graddegree, gpa, experience): status = "Not hired" if (graddegree == 'y'): if (gpa >= 3.7): status = "Hired" if (experience >= 5): status = "Hired" print "Employment status: ", status

28 Review: What Decision Making Mechanisms Are Available/When To Use Them Construct If If-else Compound decision making Nested decision making When To Use Evaluate a Boolean expression and execute some code (body) if it s true Evaluate a Boolean expression and execute some code (first body) if it s true, execute alternate code (second body) if it s false More than one Boolean expression must be evaluated. The outer Boolean expression must be true before the inner expression will even be evaluated. Common Errors 1. Inconsistent indenting (remember that you should only indent the body of something).

29 Common Errors (2) 2. Forgetting to load your program before trying to run it. Common Errors (3) 3. Your program cannot find a file that it needs. It can t find the file angel.jpg to make the picture

30 Common Errors (3) 3. Problem: Your program cannot find a file that it needs. Solution: Put all the files (pictures, sounds, videos) in the same folder where you put JES. Example Common Errors (4) 4. Forgetting required parts of the program Forgetting the word def and/or the name of the program (Erroneous version) print hello (Fixed version) def programname (): print hello Forgetting the brackets and/or the colon: def programname print jello

31 Common Errors (5) 4. Forgetting required parts of the program Forgetting the quotes when displaying a string of characters: def programname (): print hello Mismatching quotes: def programname print jello Common Errors (6) 5. Typing in your JES program using a word processor or other program that allows for powerful text formatting. These errors can be very tough to find and fix because there is no visible error in your program ( computers suck! )

32 You Should Now Know How to create and run a program using JES How the print statement displays information What are variables and how to use them in a program How to load and display images in a program How to get input as a program runs The way in which decision making works with computer programs How to write and trace (determine the execution and output of) programs that employ: if, else-if, compound decision making and nested decision making mechanisms